Ultraviolet activated antimicrobial surfaces

ABSTRACT

The invention is directed to an ion plasma deposition (IPD) method adapted to coat polymer surfaces with highly adherent antimicrobial films. A controlled ion plasma deposition (IPD) process is used to coat a metal or polymer with a selected metal/metal oxide. Exposing the coated surface to ultraviolet light significantly improves the antimicrobial properties of the deposited coatings.

This application is a continuation of U.S. application Ser. No.11/542,531, filed Oct. 3, 2006, now U.S. Pat. No. 8,623,446, whichclaims benefit of U.S. Provisional Application Ser. No. 60/776,537,filed Feb. 25, 2006, which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to light induced activation of metal coatedsurfaces and in particular to the enhancement of antimicrobialproperties of selected metal/metal oxide coated surfaces.

2. Description of Background Art

Metallic silver, silver oxides, and silver salts are highly effectiveantimicrobials which control infection by killing bacteria and virusesat wound sites. Silver ions block infection by forming insolublecompounds within the cell walls, blocking respiratory chains, andbinding and denaturing bacterial DNA, thereby preventing replication.Silver-based biocides have also shown activity against decay fungi, somecommon molds and some insects due to interference with microbes in theinsect gut (Dorau, et al., 2004).

Ionic silver is recognized as an effective bactericide at levels ofabout 0.1 μg/L while fungicidal activity requires levels on the order ofabout 1.9 μg/L (Joyce-Wohrmann and Mustedt, 1999). Silver ions disruptmicrobial cell walls and can also damage cell receptors by bindingmetabolically ineffective compounds to cell pathways. To maintaineffectiveness against bacterial growth, silver ions must be releasedcontinuously at effective levels in order to compensate for decrease ineffective concentration due to these binding interactions. On the otherhand, release of excessively high concentrations of silver can harmhealthy mammalian cells so that release profiles need to be taken intoconsideration when antimicrobial coatings are manufactured.

Silver exhibits antimicrobial activity against most pathogens and theredo not appear to be any reports of allergic reactions by patients(Russell and Hugo, 1994). Silver based coatings thus would appear to becandidates for use on surfaces of implanted medical devices in view ofthe tendency of in vivo devices to harbor serious infections.Applications of silver/silver oxide coatings have included hydrogelsimbedded with silver compounds, wet chemistry using silver salts andantimicrobial compounds, and plasma vapor deposited surfaces of silver,cast silver, and cryogenically applied silver.

Unfortunately, medical devices and implants are ideal surfaces forprimary bacterial adherence and biofilm formation. Valves and cathetersfor example provide hard surfaces in warm, moist, nutrient-richenvironments. Biofilms, once formed, are very difficult to eradicate.Over 1,500-fold concentrations of an antimicrobial agent may be requiredto kill bacteria established in a biofilm compared to the amountrequired for treatment of free floating or planktonic forms of bacteria.

A recent upsurge in antibiotic resistant bacteria has again focusedattention on the antimicrobial properties of silver and silver oxide.While some studies suggest that silver-protected surfaces on medicaldevices and implants may well be a preferred method of fightinginfection, practical and long-term effective coating methods have yet tobe developed (Tobler and Warner, 2005).

Most hospital-acquired bloodstream infections are associated with theuse of an intravascular device, such as central venous catheters.Catheter-associated bloodstream infections occur more often in intensivecare unit (ICU) patients than in ward patients. The mortality rateattributable to bloodstream infections in surgical ICUs has beenestimated to be as high as 35%. ICU-acquired bloodstream infectionsaccount for an estimated $40,000 increase in costs per survivor and anestimated $6,000 increase in hospital costs. (CDC Publication, 2001)

There are at least two important considerations in developingantibacterial coatings for use in medical implants. A recurring problemwith silver-based coatings is flaking, peeling, or sloughing of silverfrom the surface of the coated substrate. Release of high levels ofsilver ions for an extended period of time can cause localized celldeath, or necrosis. This particular problem, for example, caused St.Jude Medical to withdraw a sewn-in silver heart valve cuff from themarket in 2001 when it appeared that a silver/silver oxide coating on avalve cuff prevented proper healing. [FDA Enforcement Report 000635,Mar. 29, 2000]

Even when silver-based coatings on medical devices are sufficientlyadherent to avoid causing cell damage, the antimicrobial effects may beweak and/or sustained for only short periods of time. Medical implants,for example, tend to be a focus for infections and therefore wouldbenefit from antimicrobial coatings that maintain activity for longperiods of time without toxicity to normal cells.

Efforts have been made to produce medically acceptable antimicrobialcoatings on medical devices. The most commonly used coating processesare sputtering, ion beam assisted deposition (IBAD), and dip processes.While there are other, less commonly employed techniques, none of thesecommercially used methods has provided a coating that is both stable andantimicrobially resistant for relatively long periods of time. Thedisadvantages of these processes are briefly summarized.

Sputtering and IBAD methods are similar except that IBAD additionallyemploys an ion beam that provides a more dense coating. In the IBADprocess, ions are accelerated toward a target of antimicrobial materialsuch as silver. When the ions hit the target, individual silver atomsare “knocked-off”. The silver atoms react with oxygen in the plasma andare directed to the substrate and deposited. Problems with thistechnique include controlling the percent reacted to form AgO (theantimicrobially active form of silver), scalability, and, of mostconcern, lack of good adhesion.

Consistently good adhesion is one of the more frequently encountereddifficulties when coatings are produced by sputtering. Sputtering is alow energy process compared to other methods such as ion plasmadeposition. Because of this, incoming ions do not have sufficient energyto securely implant into the surface. In attempts to solve this issue,sputtering of an antimicrobial coating usually requires a seed layer ona substrate surface to achieve even moderate adhesion. Under staticconditions, sputtering may produce an acceptably adherent film, but ifthe substrate is twisted, bent or exposed to bacteria in vivo, asencountered with soft tissue repair devices, the coating has a highprobability of de-lamination and subsequent release of metal particlesinto the body. Silver particles are a serious problem because largeamounts of silver concentrated in one area can cause necrosis.

Controlling the actual percent of AgO can also pose a significantproblem with sputtering methods because in order to act as an effectiveantimicrobial, coatings need to consist of a large percentage of AgOversus Ag₂O. The generation of singlet oxygen is also thought to beimportant and has been known for years to provide antimicrobial activitydue to it's free radical nature (Kumar, et al., 2005).

Scalability is also a consideration with sputtering processes whencommercial quantities of coated devices are manufactured. Even whenadhesion is not a significant consideration, cost reduction can only berealized by way of scalability. The sputtering process does not lenditself to large scale production, which requires complex fixturing,small throwing power, because parts need to be in close proximity to thetarget, and because of limitations on target size. Sputtering is anextremely slow process that has a typical deposition rate of angstromsper minute. This leads to long processing times per deposition cycle, inaddition to necessary post processing to convert the non-reactive Ag₂Oto AgO. The area that can be treated at any one time is typicallylimited to 20-100 square inches. For these reasons, it is not onlyeconomically inhibiting to scale up the sputtering process, it is inpractical terms physically impossible.

Dip processing is another method of depositing an antimicrobial, whethersilver or non-silver based, onto the surface of medical devices. Theprocess of depositing a liquid based coating onto a substrate iscomplicated. The major problems with this technique are identificationof a soluble antimicrobial agent with long-lasting activity, andavoidance of uneven adherence of the agent to the substrate.

Uneven coatings on a substrate surface are generally unacceptable. Withdip processes, wetting of the surface is random and spotty at best. Thisleads to areas that lack any antimicrobial coating and are a breedingground for infection and biofilm formation.

Some attention has been devoted to modifying surfaces of antimicrobialcoatings in the hope of increasing antimicrobial activity. Ion beamshave been used to carve textures into surfaces on implants,hydrocephalic shunts, percutaneous connectors, and orthopedicprostheses. The patterns can be holes, columns, cones, or pyramids assmall as one μm. These added patterns have been described as increasinga device's surface area 20 times and therefore increasing antimicrobialactivity of deposited coatings, as suggested in U.S. Pat. No. 5,383,934.

Deficiencies in the Art

Deposition of antimicrobial materials is commonly limited to only a fewmethods for producing silver and silver oxide coatings. Each of thesemethods has serious disadvantages and none has been developed toefficiently produce the highly adherent, and evenly distributedantimicrobial films required for use on surfaces of medical devices andinstruments. Current state of the art processes, such as sputtering, dipand ion beam assisted deposition (IBAD), produce coatings with limitedadhesion to flexible substrates. Multiple layers of base coatings addedto provide adhesion not only increase processing time and costs but alsoincrease thickness, which may not be desirable.

The need for antimicrobial coatings in the medical device market is wellknown, especially for antimicrobial films that have broad activity overrelatively long periods of time. Where medical devices are used, thecoatings must also meet safety standards for in vivo use.

SUMMARY OF THE INVENTION

The present invention particularly addresses the problem of low activityin antimicrobial coatings, in addition to related problems ofinefficient coating processes, and poor substrate adhesion ofantibacterial coatings. Highly antimicrobially active coatings thatresist flaking and peeling from substrate surfaces can be produced bythe disclosed process which utilizes ion plasma deposition (IPD) incombination with ultraviolet (UV) light.

Coatings with surprisingly improved antimicrobial activities have beenobtained by exposing the surface of controlled IPD deposited metalcoatings to ultraviolet light. An entirely unexpected finding was thatseveral highly adherent metal coatings which initially showed little orno antimicrobial activity could be activated when exposed to ultravioletlight in the 200-400 nm range; in some cases exhibiting antimicrobialactivity only after exposure to ultraviolet light.

The invention is in part based on the development of an IPD-based methodthat produces predictable coating structures that have excellentadhesion, making these coatings particularly desirable for use onimplanted medical devices. The method provides antimicrobial coatingsthat can be deposited in multiple layers of antimicrobial orantimicrobially-activated materials on metal and non-metal substratesfor use in implants such as valves and indwelling catheters. The layerscan be relatively thin, for example in the 100 nm range, so thatproduction cost is reduced without sacrificing desired antimicrobialactivity.

A modified IPD/UV method has been developed for preparing antimicrobialmetal coatings that have significantly enhanced antimicrobial activity.The coatings are particularly adaptable for use on devices and materialsused in medical applications. The coatings do not flake or peel, as iscommon with coatings produced by electrodeposition or magnetronsputtering. Antibacterial activity is maintained on coatings applied topolymers and various metals because flaking and peeling are not aproblem. Moreover, the coatings exhibit significantly improvedantibacterial activity compared with currently available antimicrobialcoatings.

The antimicrobial coatings of the present invention can be applied tometals, and to polymers, which are preferred materials for medicaldevices such as catheters, stents, and plastic implants.

The invention provides coatings that are uniquely suitable for use onmedical devices for use in the human body or veterinary applications.The IPD/UV method for producing the coatings is economical and provideshigh quality coatings.

Definitions

PVD is thin film deposition process in the gas phase in which sourcematerial is physically transferred in the vacuum to the substratewithout any chemical reactions involved. This type of depositionincludes thermal evaporation electron-beam deposition and sputteringdeposition. The IPD process is a sub-segment of physical vapordeposition.

Macros and macroparticles refer to particles larger than a single ion.Small macro-particles refer to particles from two atoms to approximately100 nanometers (alternatively, nano-particles). Medium macro-particlesrefer to particles from 100 nanometers to about 1 micron. Largemacro-particles refer to particles larger than 1 micron.

Antimicrobial refers to the ability of a compound to destroy microbes,prevent their development, or inhibit their pathogenic action and asused herein is intended to apply to bacteria, yeast and other fungi.

IPD, as used in the context of experiments and methods described herein,refers to an ionic plasma deposition process that uses a modifiedcontrolled cathodic arc discharge on a target material to create highlyenergized plasma. IPD differs from normal cathodic arc processesdescribed by others in that the deposition of particle size is highlycontrolled.

The term about as used herein is intended to indicate that a specifiednumber is not necessarily exact but may be higher or lower within a 10%range as determined by the particular procedure or method used.

The term “a” as used in the claims is not intended to limit to a singlespecies.

As used herein, “substantially free” does not necessarily mean entirelyfree; rather that the amount of material present will not significantlyaffect properties claimed for the absence of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch of the IPD apparatus. Target material (1), substratebeing coated (2), mechanism for moving the substrate closer and furtheraway from the target (3), vacuum chamber (4), power supply for thetarget (5).

FIG. 2 is another embodiment of the IPD apparatus. Target material (1),substrate being coated (2), mechanism for moving the substrate closerand further away from the target (3), vacuum chamber (4), power supplyfor the target (5), device to control arc speed (6).

FIG. 3 is an example of the setup for a modified IPD/UV apparatus fordepositing an adherent highly antimicrobial coating on a substrate. Theapparatus incorporates the features of FIG. 1 and/or FIG. 2 with asource (7) to input ultraviolet light at selected wavelengths.

DETAILED DESCRIPTION

The invention is directed to the use of ultraviolet radiation incombination with highly controlled IPD conditions to achieve stablehighly antibacterial coated substrates. It is the combination of an IPDmethod for macroparticle deposition and the discovery of the use ofspecific wavelengths of ultraviolet (UV) light that provides the uniqueaspects of the present invention. The significant improvement inantimicrobial activity of surface coatings is based on the structuralfeatures of the deposited materials, the method with which the IPD iscontrolled, and ultraviolet light activation of the coating surface.

The antimicrobial coatings prepared by the new method may be depositedfrom any of a number of metals, or combinations of metals that aresuitable for ion deposition. Elements include those having an atomicnumber greater then 21, and a density greater then 4.5 g/cm², e.g., Ti,Zr, Cr, Co, Ni, Mo, Pd, Ag, Hf, Ta, W, Ir, Pt, Sn, Bi, Zn, Au, andalloys and compounds of these metals. For commercial biologicalapplications, use of silver, copper, gold, titanium and hafnium arepreferred metals. Target materials may also include AgO, TiO, TiO₂, CuO,HfN and higher oxidation forms of silver, copper, and titanium, whichmay in some cases be more highly antimicrobial compared to lower oxidesof these metals. The disclosed IPD-UV process provides highly conformed,adherent, thin, highly antimicrobial coatings, using tunable controlsthat provide antimicrobial activities within ranges that are typicallynecessary in medical applications.

Some deposited metals, such as surface film deposited titanium dioxide(TiO₂) do not exhibit antimicrobial activity when deposited byconventional deposition or standard plasma arc deposition. Thedeposition/surface activation method described herein provides anantimicrobially active surface, as demonstrated in the Examples usingthe IPD/UV process. An initially inactive deposited TiO₂ surface can beactivated to an antimicrobially active surface by the UV process.

Highly adherent antimicrobial coatings have been produced using amodified ion deposition method. The process utilizes a controlleddeposition system in combination with substrate exposure to ultraviolet(UV) light.

The disclosed modified IPD process itself generates a significant levelof UV photons and high-energy ions when oxygen is present in the system.The spectral profile of the UV energy emitted from the source (target)is determined by the specific metal(s) or alloy(s) used. A high-energycoulomb explosion excites diatomic oxygen (O₂) gas injected into thechamber with a broad range of UV energy, causing the two oxygen atoms ofground state oxygen to dissociate, yielding singlet oxygen. Ground stateoxygen is a triplet where at least two electron orbits are unpaired andparallel (Foote, 1995). Singlet oxygen is a singlet molecule (O*) thathas undergone an electron spin inversion to pair all the electrons inthe oxygen atom. Some of the differences in bonding properties arisebecause triplet oxygen is paramagnetic while singlet oxygen is slightlymagnetic.

Molecules whose outermost pair of electrons have parallel spins,symbolized by ↑↑, are in the “triplet” state; molecules whose outermostpair of electrons have anti-parallel spins, symbolized by ↑↓, are in the“singlet” state. Ground-state oxygen is in the triplet state, indicatedby the superscripted “3” in ³O₂; its two unpaired electrons haveparallel spins, a characteristic that, according to rules of physicalchemistry, does not allow them to react with most molecules. Thus,ground-state or triplet oxygen is not very reactive. However, tripletoxygen can be activated by the addition of energy, and transformed intoreactive oxygen species.

Relative intensities of the spectral lines observed for any elementdepend upon the light source and excitation conditions. Thus, even ifthe relative intensities observed in a particular experiment areadjusted to correct for the wavelength dependence of the sensitivity ofthe spectrometer and detector, the intensities will in general bedifferent from relative intensities from previous observation ortabulated in a compilation such as illustrated in Tables 1-3.

Ultraviolet radiation at wavelengths shorter than 242 nm splitsmolecular oxygen into atomic oxygen. When the energetically excitedindividual oxygen atoms encounter molecular oxygen, ozone (O₃) can formfrom the bonding of three-oxygen molecules. Shorter wavelengths in theUV region match several levels of singlet oxygen excitation, resultingin unique modifications to the electron orbit structure. At higherenergy levels it is also possible to excite electrons from the oxygenorbital into higher levels, giving rise to efficient absorption in therange of 180-200 nm, the “Schumann-Runge bands.”(http://earthobservatory.nasa.gov/Library/ChemistrySunlight/).

Each target used in the IPD process has its own spectral absorption andemission profile and is capable of generating energy within the UV rangeto provide the required energy for singlet oxygen formation. Listedbelow are representative vacuum emission spectral lines associated witha representative group of refractory metals. As shown, silver, andcopper arcs yield UV in the Schumann-Runge bands, while titanium has ahigher UV range which is still sufficient to yield singlet oxygen. Thefrequencies shown have been selected from the first excitation level ofthe atom and are shown as relative intensities. Less prominentwavelengths are not listed and relative intensities are shown only toindicate the stronger emissions in the near UV range of 160 nm to 140 nm(http://physics.nist.gov/cgi-bin/AtData/main_asd).

TABLE 1 Emission spectra from a silver arc in vacuum. SILVER 1Wavelength Vac. (nm) Relative Intensity 165.152 60 165.210 50 170.927 50184.771 20 206.183 200 207.051 100 231.027 30 237.574 50

TABLE 2 Emission spectra from a copper arc in vacuum COPPER 1 WavelengthVac. (nm) Relative Intensity 165.532 30 168.809 30 169.108 30 170.384 30171.336 50 172.566 50 174.157 50 177.482 200 182.535 100 216.577 1300217.962 1600 218.240 1700 220.027 1700 220.044 1300 222.639 2100 229.4552500 230.383 1000 239.336 2500

TABLE 3 Emission spectra from a titanium arc in vacuum. TITANIUM 1Wavelength Vac. (nm) Relative Intensity 227.7401 130 228.0669 190230.0567 150 230.3457 140 230.6397 190 238.5246 35

Activation of singlet oxygen is inherent in the IPD process due to thehigh energies involved. For some materials such as silver oxide, this issufficient to impart limited antimicrobial activity by increasing theamount of antimicrobially active oxide (AgO). For other depositedsurfaces, such as CuO, TiO, or TiO₂, another mode of activating thesinglet form can be used to obtain antimicrobial activity. It has beendiscovered that using selected wavelengths of UV light will excite thesemetals and metal oxides, which elevates oxygen to the singlet state,thereby creating a coated surface with new or enhanced antimicrobialproperties.

The IPD/UV process can include up to at least 20% more diatomic oxygenor nitrogen into the coating surface compared to traditional IPDdeposition. Oxygen or nitrogen inclusion is enhanced by first creatingan oxygen or nitrogen rich plasma before introducing ultraviolet lightinto the plasma. This causes diatomic oxygen or nitrogen to incorporateinto the substrate surface in a stable form. This results in enhancedantimicrobial activity due to the presence of singlet oxygen or nitrogenwhich is produced by the dissociation of diatomic oxygen or nitrogen.

It was recognized that an IPD process could be modified to provide anumber of advantages over other methods commonly used for coatingprocesses. Several basic features of a plasma arc process have beenmodified and exploited in developing the IPD/UV process. Unique coatingscan now be prepared which have increased surface area due to controlledparticle size and significantly increased antimicrobial activity.Several metal/metal oxide coated substrates in addition to silver/silveroxide have been prepared and demonstrated to have new or enhancedantimicrobial properties.

Plasma deposition processes release molecules from the target whichdeposit on a target surface as various sized clusters and individualatoms. The predominant trend in coating processing has been to adjustconditions to reduce the density and number of macro particle depositionin order to produce cleaner and more uniform films. Conventional wisdomin the industry has been that macro-particles in general are deleteriousto the quality of deposited films. In contrast, the present inventionclearly illustrates the advantages of increasing macro particledeposition, not only on metals but also on plastic substrates, in orderto obtain adherent films that can be surface irradiated to enhanceantimicrobial activity. It has also been found that, in general, highermacro particle deposition rates result in lower temperature depositions,while lower deposition rates result in higher temperature depositions.Higher deposition rates are thus advantageous in coating thermosensitive materials such as certain plastics.

Arc control in the IPD process can be used for faster movement, whichwill create fewer and less dense arrays macro particles without the useof sensors or filters, or slower movement, which creates a greaternumber of more densely packed macro particles. This type of control alsoprovides the option of mixing the two modes to create moderate amountsof macro particles, or creating a near macro-free coating followed by amacro-dense coating. The amount of macro-particles can be directlyrelated to the amount of available silver that combines to form AgO, andtherefore aids in the ability to tune the duration of the efficacy ofthe coating.

Adhesion of metals onto plastics using vapor deposition processes otherthan IPD, electroplating, or electro-less plating often results in lossof some physical properties of the original substrate. For most metalsdeposited by these processes, adhesion is dependent on a strike layer oftitanium or chromium and even then, tends to delaminate if the substrateis bent, twisted or stretched. The IPD coating process, under theconditions described, imbeds into the substrate so adhesion is notaffected by subsequent mechanical stress on the substrate.

Using controlled deposition rates, IPD can be performed at lowertemperatures than most vapor deposition processes, which require apre-heat cycle and glow discharge, the pair usually resulting intemperatures exceeding 200° C. Most plastics melt well below thistemperature. The IPD process can be performed at a much lowertemperature, allowing for low melting point plastics to be effectivelycoated without adversely affecting the original substratespecifications. Such low temperature deposition is achieved bycontrolling the rate at which the metal reacts with oxygen. Making moreoxygen available for reaction in the system by inputting molecularoxygen or ozone allows the devices to stay cooler due to conductivecooling and slowing of the ions due to collisions.

IPD increases throughput up to 30 times compared to other plasma vapordeposition processes and dip processes, while at the same time achievinghigh densities and favorable antimicrobial activity. The disclosedmodified IPD process for depositing an antimicrobial coating has athroughput up to ten times greater than traditional cathodic arc.

Unlike traditional PVD and dip processes, the IPD antimicrobial coatingscan be scaled as large as necessary and still achieve high throughputwhile maintaining quality and economy of coating necessary forcommercial operations.

The IPD process provides antimicrobial coatings that otherwise are noteasily produced, or even possible in some cases, by traditional PVD.Some examples, not intended to be limiting, include silver oxide, copperoxide and hafnium nitride. Silver/silver oxide coatings have a higherantimicrobial activity when produced by the IPD method than thecomparably active but thicker coatings obtained from more expensiveprocesses; for example the magnetron sputtered antimicrobial coatingsdescribed by Burrell, et al. (1995). Thinner coatings, and thereforeshorter processing times, can be applied using the presently disclosedIPD method to achieve at least the same antimicrobial activity as in thethicker films.

Typical PVD and electroplating are line of sight deposition methods.Because of this, it is difficult to coat complex and oddly-shapeddevices without complicated fixtures, and even with the correct fixture,it may not be possible to evenly coat the devices. The modified IPDprocess provides non-line of sight coating but still maintains theantimicrobial qualities of the coating without the use of complicatedfixtures because the coatings are readily conformed to the part.

IPD coating rates are extremely fast. With the relatively short time inthe plasma to achieve a desirable antimicrobial coating, the temperatureof the substrate does not rise very fast or very high. This gives anadvantage over other coating methods that require cooling steps or longdeposition cycles to achieve the same antimicrobial properties. The fastcoating rates are also commercially attractive because higher productthroughput, up to 10-fold, than with sputtering, electroplating, or IBADprocessing is possible.

The new IPD/UV methods and coatings introduce several improvements toexisting technology, including use of more/less macro particles tocontrol the duration of antimicrobial coating activity, use of morereactive oxygen to increase the ratio of active to inactive silver oxide(AgO/Ag₂O), use of selected wavelengths of UV light during deposition toactivate singlet oxygen, and the ability to lay down a thinner coatingthan current art allows while maintaining equivalent antimicrobialproperties.

EXAMPLES

The following examples are intended to illustrate the invention and/orto provide background and are not intended to be limiting.

Methods

Antibacterial activity of the deposited coatings was tested using a zoneof inhibition test (ZOI). Mueller Hinton agar was dispensed into Petridishes. The agar plates were allowed to surface dry prior to beinginoculated with a lawn of Staphylococcus aureus ATCC #25923. Theinoculant was prepared from Bactrol Discs (Difco M.) which werereconstituted per the manufacturer's directions. Immediately afterinoculation, the coated materials to be tested were placed on thesurface of the agar. The dishes were incubated for 24 hr at 37° C. Afterthe incubation period, the ZOI was measured and a corrected ZOI wascalculated as follows: corrected ZOI=ZOI minus the diameter of the testmaterial in contact with the agar.

Examples 1-3 are provided as background comparisons for theantimicrobial coatings prepared as previously reported by others. Thedeposition methods of the present invention (see Examples 4, et seq.)are based on a modified IPD process while the previously publishedcomparison procedures in examples 1-3 utilize sputtering deposition.

Example 1 Antibacterial Activity of Sputtered Silver Coating on Latex

This example was performed in accordance with the coating and testingprocedures described in U.S. Pat. No. 5,454,886 (the '886 patent). Themethod and testing were performed in accordance with the proceduredetailed in Example 6 of the '886 patent.

Silver metal was deposited on 2.5 cm sections of a latex Foley catheterusing a magnetron sputtering facility. Operating conditions were asfollows; the deposition rate was 200 A° per minute; the argon workinggas pressure was 30 m Torr; and the ratio of temperature of substrate tomelting point of the coating metal silver, T/Tm was 0.30. In thisexample the angles of incidence were variable since the substrate wasround and rough. That is, the angles of incidence varied around thecircumference and, on a finer scale, across the sides and tops of thenumerous surface features. The anti-microbial effect was tested by azone of inhibition test, identical to the test described in Example 1 ofthe '866 patent with S. aureus ATCC accession number 25923 as the testorganism.

The zone of inhibition (ZOI) was less than 1 mm around the cathetertubing in contrast to the 16 mm ZOI reported in the '886 patent.

Example 2 Antibacterial Activity of Sputtered Silver Coating over Teflonon Latex

This example follows the procedures reported for preparing aTeflon®-coated latex catheter coated by DC magnetron sputtering inaccordance with Example 7 in U.S. Pat. No. 5,454,886. Antimicrobialtesting was performed with S. Aureus as described.

A Teflon coated latex Foley catheter was coated by DC magnetronsputtering 99.99% pure silver on the surface under the followingconditions: 0.5 kW power, 40 mTorr Ar/O₂, 20° C. initial substratetemperature, a cathode/anode distance of 100 mm, and a final filmthickness of 300 nm. The working gases were commercial Ar and 99/1 wt %Ar/O₂.

The anti-microbial effect of the coating was tested by a ZOI asdescribed in Example 7 of the '886 patent. Mueller Hinton agar wasdispensed into Petri dishes. The agar plates were allowed to surface dryprior to being inoculated with a lawn of Staphylococcus aureus ATCC#25923. The inoculant was prepared from Bactrol Discs (Difco, M.) whichwere reconstituted as per the manufacturer's directions. Immediatelyafter inoculation, the coated materials to be tested were placed on thesurface of the agar. The dishes were incubated for 24 hr. at 37° C.After the incubation period, the zone of inhibition was measured and acorrected zone of inhibition was calculated (corrected zone ofinhibition=zone of inhibition-diameter of the test material in contactwith the agar).

Uncoated samples showed no zone of inhibition. The coated sample showeda ZOI of less than 1 mm in contrast to the corrected ZOI of 11 mmreported in Example 7 of the '866 patent for the catheters sputtered inthe 99/1 wt % Ar/O₂ using a working gas pressure of 40 mTorr.

Example 3 Sputtered Antibacterial Silver Coating

This example was performed in accordance with the procedure described inExample 11 in the '866 patent. Conditions used for this exampleincluded: RF magnetron power of 0.5 kW, 40 mTorr pressure, 100 mmanode/cathode distance, and 20° C.

When a working gas of argon and 20 wt % oxygen was used to sputteranti-microbial coatings under the conditions listed above, the zones ofinhibition ranged from 0 to 2 mm, in contrast to the ZOI of 6 to 12 mmreported in the '866 patent Example 11.

Example 4 Control of Macro Particle Density in IPD Coatings

Control of the distance/current relationship in IPD proceduresdetermines amount and size of the deposited macro particles. The closerthe substrate is to a source (target), the more macro-particles will bepresent on the substrate. Macro particles evaporate as they are ejectedfrom the target. Therefore, the longer the time of flight, the morematerial is evaporated from the particle. Macro particle density canalso be controlled by the current because either a higher current orlimiting the current to a level that occurs just before an arc splittends to cause more and larger macro particles.

A motorized unit capable of moving a substrate closer to and fartheraway from the target (cathode) was used to initially deposit asubstantially macro-free film. This provides a base coat with excellentadhesion properties. A more macro particle dense film is then depositedby positioning the substrate closer to the target. The macro densesurface has enhanced antimicrobial activity by comparison to filmshaving a relatively macro particle free surface. FIG. 1 illustrates anIPD apparatus indicating how the position of the substrate with respectto the target can be moved. The IPD is conducted in an oxygenatmosphere.

Macroparticle size can also be controlled by use of a variable IPD powersource, which can be configured to sufficiently slow (or accelerate) thespeed of the arc. The traveling speed of the arc is directly related tothe amount of macro particles produced. Essentially, slowing the speedof the arc on the surface of the target (cathode) will cause it toproduce more macro particles, which can be used to increase the macroparticle density. Conversely, increasing the speed of the arc on thecathode will decrease production of macro particles, thereby providingmore high energy ions that can be embedded into the surface of thesubstrate to produce better adhesion. FIG. 2 shows an IPD apparatussetup with control of arc speed and substrate position with respect tothe target.

Increase and decrease travel speed of the arc can be controlled by anappropriate device such as the mechanical switch described in U.S. Pat.No. 6,936,145. The switch toggles current to two or more points on thetarget and is an example of one method of speed control, although othermethods of control can be used. The increase and decrease of arc speedallows the deposition (without internal movement) of a substantiallymacro-free film for adhesion followed directly by a macro dense film bymanipulation of the arc speed.

Example 5 Increasing AgO in Antimicrobial Films

An advantage of the combined IPD/UV method is that the IPD processitself can be adjusted so that more oxygen and metallic ions can be madeavailable for combination in forming an antimicrobial film. Whenactivated by ultraviolet light, the IPD films have significantlyenhanced antimicrobial activity. The control of the plasma arc speed(see FIG. 2) can provide substantially 100% ionized oxygen plasma whenthe IPD is conducted in an oxygen atmosphere. The percent of singletoxygen in the plasma can be further enhanced by injecting ozone insteadof diatomic oxygen into the system. The presence of oxygen, in additionto the ability of IPD to create a highly ionized metal stream from thetarget means that more AgO is created films than by other methods and,in combination with exposing the deposited surface to UV light, resultsin significant enhancement of antimicrobial properties of metal/metaloxide coated surfaces.

Example 6 Ultraviolet Activation of IPD Deposited Ag, Ti and Cu Coatings

The additional use of UV light during or after the deposition activatessinglet oxygen in silver/silver oxide deposited coatings. Silver oxidetends to relax to Ag₂O, the more stable form of silver oxide. UV lightcan also be used to activate Ti/TiO₂ and Cu/CuO coatings. Forsilver-based surfaces, UV light directed into the system convertsdeposited Ag₂O to AgO. UV light can be supplied from a source within thevacuum chamber (FIG. 3) or from an outside source after the coatedsubstrate is removed from the chamber.

Glass substrates were coated with 100 nm of a combination of Ag, AgO,and Ag₂O using the IPD process as described in Example 4. Samples weretested by a zone of inhibition (ZOI) test on tryptic soy agar with S.aureus. Half the coated samples were incubated at 37° C. with no lightexposure; the other half were incubated at 37° C. after exposure to UVlight in the range of 200 to 400 nm. After a 24 hr incubation, thesamples not exposed to UV showed a ZOI of up to 6 mm. The samplesexposed to UV light showed a zone of inhibition of up to 12 mm, seeTable 4.

Glass substrates were coated with 100 nm of a combination of Ti, TiO andTiO₂ using the IPD process described in Example 4. Antibacterialproperties of the coated samples were tested using zone of inhibitiontesting on tryptic soy agar with S. aureus. Half the samples wereincubated at 37° C. with no light exposure, the other half wereincubated at 37° C. after exposure to Black Light Blue (BLB) in thewavelength range of 300-400 nm. After a 24 hr incubation, the samplesnot exposed to BLB showed no zone of inhibition. The samples exposed toBLB showed a zone of inhibition of up to 12 mm, see Table 4.

Glass substrates were coated with 100 nm of a combination of Cu, CuO,and Cu₂O using the IPD process. The samples were tested by ZOI ontryptic soy agar with S. aureus. Half the samples were incubated at 37°C. with no light exposure, the other half were incubated at 37° C. afterexposure to UV light in the range of 200 to 400 nm. After a 24 hrincubation, the samples not exposed to UV showed no ZOI. The samplesexposed to UV all showed a ZOI. The ZOI was enhanced two-fold for theAg-based coatings. The UV treatment on the Ti and Cu-based coatingsproduced antibacterial activity comparable to the Ag-based coatingswhere, unlike the silver coatings, no activity was observed before UVtreatment. See Table 4.

TABLE 4 Metal Coating Light Combination Thickness Wavelength Microbe ZOI(24 hr) Ti, TiO, TiO2 100 nm None S. aureus None 100 nm None C. albicansNone 100 nm 300-400 nm S. aureus 12 mm 100 nm 300-400 nm C. albicans 12mm Cu, CuO, Cu₂O 100 nm None S. aureus None 100 nm None C. albicans None100 nm 200-400 nm S. aureus 12 mm 100 nm 200-400 nm C. albicans 12 mmAg, AgO, Ag₂O 100 nm None S. aureus  6 mm 100 nm None C. albicans  6 mm100 nm 200-400 nm S. aureus 12 mm 100 nm 200-400 nm C. albicans 12 mmThe ZOI was calculated as indicated above. This corrected ZOI is notcomparable to the calculated ZOI data reported in Burrell, et al. (1995)which were measured by subtracting the dimensions of the substrate fromthe dimensions of the observed zone of inhibition.

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What is claimed is:
 1. A method for producing a highly adherent,antimicrobial coating, comprising: depositing a first substantiallymacroparticulate-free metal coating on a selected substrate; depositinga second macroparticulate dense metal coating to form a surface layer onsaid first coating; and exposing the surface layer to ultraviolet lightin the range of 120-400 nm during deposition of the first and secondmetal coating; wherein said metal coating is deposited by ionic plasmadeposition in the presence of an oxygen atmosphere controlled to adjustmacroparticle density in the first and second layer.